This invention relates to an apparatus and method for monitoring the stitching quality of sewing machines and, in particular, to detecting skipped stitches for Class 301 lockstitch sewing machines.
With the clothing industry becoming increasingly automated, there is a need for systems that monitor and regulate the functions and output of high speed sewing equipment. Certain of these systems are utilized to monitor the stitching of sewing machines to detect skipped stitches in apparel manufactured by Class 301 lockstitch sewing machines. The Class 301 lockstitch is employed in a wide range of areas within the apparel industry because it provides a fast, economical, and strong stitch.
In the general use of lockstitch type 301 sewing machines, improper stitches may from time to time be introduced in a workpiece. Generally, improper stitches may have the form of malformed stitches or skipped stitches. There are many causes of skipped stitches. Skipped stitches can develop from improper synchronization between the active elements within the sewing machine and the needle and bobbin thread loops. Normally, the bobbin hook catches the needle loop and brings the needle thread around the bobbin to form a lockstitch. Skipped stitches are most often formed when the bobbin hook fails to grasp the needle loop.
Malformed stitches are formed when the 301 stitch is not properly set. A stitch is properly set when the needle thread and the bobbin thread interlock in the center of the workpiece. A malformed stitch occurs when the stitch interlocks at either the top of the workpiece or the bottom of the workpiece.
In the prior art, skipped stitch detection systems are based upon monitoring the tension of the needle thread. As an example of this system, in U.S. Pat. No. 4,102,283 (Rockerarth et al.) the loss of thread tension generally is said to correspond to a skipped stitch, and this reduction in normal thread tension triggers a sensing device. The sensitivity of these systems ranges from complete loss of thread tension, for example due to the thread breaking, to sensing a momentary reduction in normal thread tension.
Other systems are based upon monitoring thread consumption, and may correlate thread consumption with total number of stitches, to detect a skipped stitch. As an example of this system, in U.S. Pat. No. 3,843,883 (DeVita et al., Oct. 22, 1974) a monitor is used to measure thread consumption which is then compared to a predetermined standard of thread use, deviation from which activates an output signal.
Another system used for detecting skipped stitches in a lockstitch type 301 sewing machine is disclosed in UK Patent Application No. GB 2008631. That system involves monitoring the length of a seam as compared with the upper thread consumption required to produce the seam. Actual thread consumption is then compared against a predetermined consumption value, any difference of which corresponds to an improperly formed seam. However, the difference in upper thread consumption between correct stitches and skipped stitches is not always substantial enough to be reliable in fast-rate sewing machines. This is best demonstrated when two pieces of thin fabric are being sewn together. Generally, measurements of the difference in thread consumption per stitch includes the thickness of two plies of fabric (assuming the stitch is set at center). For example, letting stitch length (SL)=0.125 inches, and ply thickness (PT)=0.01 inches, then the percentage decrease for a skipped stitch would be: 100 * [(2 * PT)/SL]=100 * [(2*0.010)/0.125]=16%. If thread tensions are not adjusted properly, this percent decrease could go to zero. Thus, there is a need for a direct, effective method of detecting skipped stitches in a fast-speed lockstitch type 301 sewing machine.
A primary shortcoming of the prior art is the unreliability of these systems at high sewing speeds, for example greater than 5,500 stitches per minute. DeVita states that the apparatus disclosed therein makes "mechanically possible the very high running speeds of about 2,000 stitches per minute desirable for such [lockstitch] sewing machines" (emphasis added). These systems fail to detect a momentary reduction of thread tension when the sewing machine is operating at high sewing speeds. The reduction in tension for an improper stitch at high sewing speeds tends to be less and in a range that the prior art fails to detect. As a result, these systems tend to be less reliable and thus fail to perform these functions with great accuracy.
There exists a need for better methods and systems for detecting skipped stitches that are reliable at high sewing speeds. To accommodate the advances in the clothing automation, particularly the increase in sewing speeds, it is an object of the invention to provide a simple, reliable system for detecting skipped stitches that would satisfy a substantial need in the art.